The 16660 COMEX Sea-Dweller.. A study in transition.. by Delgado and others and rescently updated
Photo by Shin-An

Below photos by delgado

The 16660 was a transitional watch for Rolex. The new case, sapphire crystal, larger helium valve meant that the new watch could be taken to depths up to an incredible 4000 feet or 1220 meters. There was also a new movement inside the case, the 3035. The new movement replaced the 1575 in the previous 1665 Sea-Dweller models.
Interestingly, the new watch also was designed to have a new dial, a glossy dial with metal rims around the markers. In the mean time, there was still some matt dials that needed to be used, so they first batch had a matt dial and the later models had the new shinny dials.
UPDATED:
The 16660 was delivered to COMEX in 1980-1984. The first 100 watches had the older style matt dials and two lines of writing, while the remaining 50 watches had the new glossy dials.
This is an example of those few 100 watches with a matt dial one..
UPDATED:
Its has been long believed that only the first 50 watches were matt dial and the rest glossy. Due to further detailed research by collectors, there is now evidence that the original batch of 100 watches from 3001-3099 had matt dials and some were replaced by service glossy dials by Rolex.

Here is compared to the legendary 1665 model that it replaced.

Especial thanks to M. Pisani for some of the background information.